1. Field of the Invention
The invention relates to a liquid crystal display device including two substrates arranged in facing relation to each other, one of which is designed to have no electrodes thereon.
2. Description of the Related Art
An active matrix type liquid crystal display device including a thin film transistor (TFT) as a switch for switching on/off of pixels presents high-quality images, and hence, is used in a lot of products such as a monitor for a desk-top type computer.
In general, a liquid crystal display device operates in either a twisted nematic mode in which liquid crystal molecules having twisted initial orientation are rotated vertically to transparent substrates, a VA mode in which liquid crystal molecules have initial orientation arranged vertical to substrates, or an in-plane switching (IPS) mode in which liquid crystal molecules are rotated in parallel with transparent substrates.
In an in-plane switching (IPS) mode liquid crystal display device, a transparent substrate on which a thin film transistor is fabricated is designed to include pixel electrodes and common electrodes alternately arranged and both having comb-teeth arranged in parallel with each other. By applying a voltage across the pixel and common electrodes, there is generated an electric field in parallel with the substrate. The thus generated electric field changes alignment direction of liquid crystal molecules to thereby control light transmissivity, that is, drive liquid crystal in a plane in parallel with the substrate. Accordingly, an in-plane switching (IPS) mode liquid crystal display device has an advantage that images can be provided with small contrast degradation and small color variance over a wide range of viewing angles, because liquid crystal molecules rotate in a plane in parallel with a substrate.
FIG. 1A is a plan view of a conventional IPS liquid crystal display device, and FIG. 1B is a cross-sectional view taken along the line 1B-1B in FIG. 1A. Hereinbelow, a structure of a conventional IPS liquid crystal display device is explained with reference to FIGS. 1A and 1B.
As illustrated in FIG. 1B, an IPS liquid crystal display device is comprised of an active device substrate 100 on which a thin film transistor (TFT) 5 is fabricated, an opposed substrate 110 on which a color filter is fabricated, and a liquid crystal layer 10 sandwiched between the substrates 100 and 110.
The active device substrate 100 includes a first transparent substrate 1, a common electrode 3, an interlayer insulating film 4, a data line 6, a pixel electrode 7, a passivation film 8, an alignment film 9, and a polarizing plate 16a. 
On the first transparent substrate 1 are formed scanning lines 2 and the data lines (signal lines) 6 such that they extend perpendicularly to each other. Thin film transistors 5 are arranged in a matrix at intersections of the scanning lines 2 with the data lines 6. In each of pixels, the pixel electrode 7 and the common electrode 3 are alternately arranged in parallel with each other.
The common electrode 3 is electrically insulated from the pixel electrode 7 and the data line 6 through the interlayer insulating film 4. The passivation film 8 covers the pixel electrode 7 and the data line 6 therewith.
The opposed substrate 11 includes a second transparent substrate 11, a black matrix layer 12 for shielding extra light, color layers 13 for displaying images having red (R), green (G) and blue (B) colors, an overcoat layer 14, an alignment film 9, an electrically conductive layer 15, and a polarizing plate 16b. 
Specifically, on the second transparent substrate 11 are formed the black matrix layer 12, the color layers 13 partially covering the black matrix layer 12 therewith, and the overcoat layer 14 entirely covering the black matrix layer 12 and the color layers 13 therewith.
The first and second transparent substrates 1 and 11 are coated at surfaces thereof with the alignment film 9. Liquid crystal molecules in the liquid crystal layer 10 are homogeneously aligned with a certain angle longitudinally of the pixel electrode 7. The first transparent substrate 1 is covered with the polarizing plate 16a, and the second transparent substrate 11 is covered with the polarizing plate 16b with the electrically conductive layer 15 being sandwiched therebetween. The polarizing plates 16a and 16b have polarization axes extending perpendicularly to each other, and one of the polarization axes is designed to extend in parallel with a direction in which liquid crystal molecules in the liquid crystal layer 10 are aligned.
In the above-mentioned IPS liquid crystal display device, a voltage is applied to the pixel electrode 7 through the thin film transistor 5 to thereby apply a horizontal electric field across the pixel electrode 7 and the common electrode 3. As a result, liquid crystal molecules in the liquid crystal layer 10 are twisted in a plane in parallel with the transparent substrates 1 and 11, thereby accomplishing control in displaying images.
A liquid crystal display device has been conventionally used mainly as a monitor of a note-type or desk-top type personal computer, but is presently used in various fields such as a television set and multi-media. With expansion in fields in which a liquid crystal display device is used, a liquid crystal display device is required to have enhanced characteristic of a viewing angle and ability to match wide chromaticity area.
With respect to a device used in a field of television, such as a monitor, there are standardized systems for transmitting image signals. As typical systems, there are National Television System Committee (NTSC) adopted by USA and Japan, and European Broadcasting Union (EBU) adopted by Europe. In order to apply a liquid crystal display device to a television, it is necessary to fabricate a liquid crystal display device to meet with one of the above-mentioned systems.
A liquid crystal display device has been conventionally fabricated so as to meet with NTSC in which a chromaticity area is about 60%. In order for a liquid crystal display device to meet with EBU in which a chromaticity area is wider than NTSC, specifically, about 70% or greater, a liquid crystal display device has to have improved optical characteristics in parts constituting a liquid crystal display device, in particular, in a color filter. To this end, it would be necessary for a liquid crystal display device to include a color filter having optimally arranged pigments.
It is known that a twisted nematic type liquid crystal display device is accompanied with a problem of defectiveness in displaying images, caused by optical characteristics of parts constituting the liquid crystal display device or performances of liquid crystal. For instance, as such defectiveness caused by a color filter, there is known defectiveness called “white non-uniformity”.
So-called “white non-uniformity” is a phenomenon in which a view recognizes non-uniformity, because light transmissivity in a part of a display area does not become zero, when a voltage is applied across electrodes for displaying black. The reason why “white non-uniformity” occurs is considered that though a voltage having been applied across electrodes should be kept constant, the voltage across the electrodes drops, because a current runs through ionic materials existing in liquid crystal.
For instance, Japanese Patent Application Publications Nos. 2001-305332 and 2000-186225 have suggested a method of preventing occurrence of “white non-uniformity”, based on the discovery that impurities existing in a color filter are transferred into liquid crystal.
Specifically, Japanese Patent Application Publication No. 2001-305332 has suggested a method of reducing impurities contained in pigment and resin. Resin of which a color filter making contact with a liquid crystal layer is composed is considered as one of ionic material sources, and hence, the Publication pays attention to a voltage retention rate and residual DC of liquid crystal, as characteristics relevant to display defectiveness caused by ionic material transferring to a liquid crystal layer from the resin. According to the suggested method, it is said to be possible to reduce ionic materials in a color filter, and prevent “white non-uniformity”.
One of factors for reducing display quality in a liquid crystal display device including a substrate on which a color filter is formed, but no electrodes are formed, such as an in-plane switching mode liquid crystal display device, is disturbance in alignment of a liquid crystal layer, caused by an electric field leaked from a scanning line and a signal line both formed on an active device substrate.
For instance, Japanese Patent No. 3125872, and Japanese Patent Application Publications Nos. 2002-323706, 11-190860, and 11-024104 have suggested a method of shielding leaking electric field.
Specifically, in Japanese Patent No. 3125872, a common electrode is formed so as to entirely cover therewith a scanning line disposed therebelow to thereby shield an electric field leaking from the scanning line.
In Japanese Patent Application Publication No. 2002-323706, common electrodes are formed at opposite sides about a data line, or a common electrode is formed above a data line to thereby prevent an electric field from leaking into a display area.
In Japanese Patent Application Publication No. 11-190860, an electrode for shielding leaked electric field is formed in the vicinity of a scanning line to thereby shield an electric field leaking from the scanning line.
In Japanese Patent Application Publication No. 11-024104, a signal line and a scanning line are covered with a transparent common electrode to thereby shield an electric field leaking from the signal and scanning lines.
Apart from the above-mentioned “white non-uniformity”, there is color non-uniformity as display defectiveness, such as “red non-uniformity” and “blue non-uniformity”.
The color non-uniformity is a phenomenon in which when a light passing through one or more color layers among color layers constituting a color filter is reduced, and resultingly, a brightness of displayed color goes unbalance, and thus, displayed images are partially colored.
In particular, such color non-uniformity is likely to occur in a liquid crystal display device having an opposed substrate on which a color filter including colorants containing red, green and blue pigments at a high concentration is formed, but no electrodes are formed is.
In accordance with the analysis conducted by the inventors, such color non-uniformity is not caused by ionic materials transferring into liquid crystal. Hence, it is not possible to prevent occurrence of color non-uniformity by the above-mentioned Japanese Patent Application Publications Nos. 2001-305332 and 2000-186225.
The above-mentioned Japanese Patent No. 3125872, and Japanese Patent Application Publications Nos. 2002-323706, 11-190860, and 11-024104 are accompanied with problems as follows.
The liquid crystal display device suggested in Japanese Patent No. 3125872 is designed to include an active device substrate having TN active matrix type TFT unlike the present invention explained later, and cannot structurally have an opposed substrate on which no electrodes are formed.
In Japanese Patent Application Publication No. 2002-323706, a scanning line is not shielded, resulting in that an electric field may leak from the scanning line.
In Japanese Patent Application Publication No. 11-190860, an electrode for shielding an electric field is arranged in the vicinity of a scanning line. However, since the electrode does not overlap the scanning line, it would not be possible to completely shield an electric field to be leaked from the scanning line.
In Japanese Patent Application Publication No. 11-024104, a signal line and a scanning line are shielded with a transparent common electrode. However, since a common electrode and the transparent common electrode are formed in separate layers, it would be unavoidable for a process of fabricating an active device substrate to become complex.
In accordance with the analysis having been conducted by the inventors, it was found out that an electric field leaking from a scanning line facilitated occurrence of color non-uniformity in an in-plane switching mode liquid crystal display device including an opposed substrate on which a color filter including colorants containing red, green and blue pigments at a high concentration is formed.
As mentioned above, color non-uniformity as one of display defectiveness in a liquid crystal display device is remarkably found in an in-plane switching mode liquid crystal display device on which a color filter is formed, but no electrodes are formed.
As a result of the analysis having been conducted by the inventors with respect to the above-mentioned problem, color non-uniformity, it was found out that since a transparent electrode such as an electrode composed of indium tin oxide (ITO) was not formed on a surface of an opposed surface, facing a liquid crystal layer, an electric field leaking from a scanning line when liquid crystal was driven reached the opposed substrate having a color filter, and hence, the electric field was applied to color layers with the result that the color layers were electrically charged, causing display defectiveness.
As explained above, color non-uniformity in display defectiveness is caused when color layers constituting a color filter are electrically charged due to an electric field leaking from a scanning line and a signal line both formed on an active device substrate.
The electrically charged color layers cause alignment disturbance in a liquid crystal layer in a pixel area with the result of reduction in a brightness, and the reduction in a brightness cause unbalance in a desired brightness in a displayed color. Such brightness unbalance is recognized by a viewer as color non-uniformity.
Japanese Patent Application Publication No. 11-194366 has suggested a liquid crystal display device including a substrate, a plurality of signal lines formed on the substrate, a plurality of scanning lines formed on the substrate to intersect with the signal lines, and pixel and common electrodes formed at intersections of the signal and scanning lines and in parallel with each other.
Japanese Patent Application Publication No. 2000-89240 has suggested an IPS mode active matrix type liquid crystal display device including a common or pixel electrode entirely overlapping a gate bus line and a gate electrode both formed below the common or pixel electrode. The suggested liquid crystal display device shields an electric field radiated from the gate bus line.